Hydraulic apparatus

ABSTRACT

This invention provides a power supply assembly suitable for a vehicle such as a fork-lift truck in which one engine provides power for propulsion of the truck and for operation of the lift and tilt service.

[ Feb. 18, 1975 United States Patent 1 1 Waters [56] References CitedUNITED STATES PATENTS i 1 HYDRAULIC APPARATUS Inventor: John HenryWaters, Cheltenham,

England 2,058,894 10/1936 2,867,091 1/1959 Orlot'fcl a1. 3,633,359l/l972 [73] Assignee: Dowty Hydraulic Units Limited,

Cheltenham, England Dec. 3, 1973 App]. No.: 420,862

Primary Examiner-Edgar W. Geoghegan Attorney, Agent, or Firm-Young &Thompson [57] ABSTRACT This invention provides a power supply assemblysuitable for a vehicle such as a fork-lift truck in which one engineprovides power for propulsion of the truck and for operation of the liftand till service.

6 6 00 2mm M 0 m 6 0 m 2 N 4 n n 0 6 u ,1. a s m U .l

[58] Field of Search 60/420, 423, 427, 431, 60/443, 444, 468 494 13Claims, 3 Drawing Figures PATENTED FEB 1 8 I975 sum 3 BF HYDRAULICAPPARATUS This invention relates to a power supply assembly to provide avariable speed variable torque drive, and is an improvement of theinvention in my'prior applicaton Ser. No: 146,105 filed May 24, 1971,which issued as US. Pat. No. 3,797,244 on Mar. 19, 1974.

In accordance with the present invention a power supply assemblycomprises a positive-displacement pump, a variable speed power sourceadapted to drive the pump, a positive-displacement motor fed withhydraulic liquid delivered by the pump, a variable by-pass to cause someof the hydraulic liquid delivered by the pump to by-pass the motor inorder to vary the motor speed and means for controlling the speed of thepower-source said means being responsive to the bypass flow and soarranged that a reduction in the by-pass flow will cause an increase inthe speed of the power source and vice versa.

The pump may be of fixed positive-displacement or of smoothly variablepositive-displacement.

The motor may be of fixed positive-displacement or of smoothly variablepositive-displacement. In the latter case the motor speed may be variedboth by variation of motor displacement and by variation of liquid flowfrom the pump to the motor.

The variable by-pass may comprise a variable dividing valve and avariable throttle valve, the dividing valve acting to by-pass part ofthe pump delivery to a by-pass circuit in the sense to maintain aconstant pressure drop across the throttle valve through which liquidflows to the motor.

The by-pass circuit may include a flow sensing means for generating apressure drop as a result of flow of bypass liquid therethrough and themeans for controlling the speed of the power source may comprise aspringloaded variable volume device connected to respond to pressuredrop across the flow sensing means for adjustment of power source speed.

The flow sensing means may comprise a restrictor or alternatively maycomprise a restrictor and a springloaded check valve in parallelconnection with one another.

The by-pass circuit may include one or more open centre control valvesand a service connected to the or each open centre valve such that theopen centre valve may either permit unrestricted flow in the by-passcircuit or may throttle the bypass circuit to direct liquid at pressureto the service. The service may be singleacting or double-acting.

The flow sensing means may be placed downstream of the open centre valveand means associated with the open centre valve may divert return flowfrom the service away from the flow sensing means. This will avoidresponse of the flow sensing means to possible high flow rates resultingfrom high return flow rates from the service which might otherwise causeexcessive reduction of power source speed.

One embodiment of the invention for use in providing the propulsion andservice on a fork-lift truck will now be particularly described withreference to the accompanying drawings, in which FIGS. 1 and 2 jointlyform a circuit diagram similar to the diagram in my said priorapplication illustrating the pump and propulsion motor control circuit,and

FIG. 3 is a circuit diagram showing the by-pass circuit and servicecontrol associated with FIG. 1.

Reference is made initially to FIG. 1. The power source for thetransmission comprises, an engine 1 which may be of any conventionaltype capable of speed adjustment, for example, a diesel engine or apetrol engine. The transmission comprises a fixed positivedisplacementpump 2 for example of the gear type, hydraulically connected to drive avariable positivedisplacement motor 3 which in turn is mechanicallyconnected to ground-engaging wheels or the like for propelling thefork-lift truck. The motor 3 may be of the swashplate type whosedisplacement is adjustable by movement of a plunger 4 relative to themotor. As shown, outward movement of the plunger 4 increases motordisplacement to a maximum. Pump 2 draws liquid from a low pressurereservoir 5 and delivers liquid at pressure to a pipe 6 which passes toa flow-dividing valve 7. The valve 7 comprises a spool valve 8 slidablewithin a cylinder 9 having three ports ll, 12 and 13 formed therein. Thepipe 6 connects to port 12. The spool member 8 includes three lands l4,l5 and 16 of which the central land 15 is somewhat narrower than theport 12 and by variation in position is adapted to divide the flow fromport 12 between the ports 11 and 13. The lands 14 and 16 define a pairof working spaces 17 and 18 at the two ends of the cylinder 9. Theworking space 18 is connected by an internal passage 19 within the spoolmember to receive the pressure within the port 11. Working space 17contains a spring 21 and receives liquid at pressure from a pipe 22through a restrictor 23. A pipe 24 carries liquid from port 11 to avariable throttle valve 25. The port 13 is connected to a by-passpassage 26 extending to the by-pass circuit shown in FIG. 3 which willbe described later in the specification.

The variable throttle valve 25 comprises a cylinder 27 having five ports28, 29, 31, 32 and 33 formed therein. A spool valve member 34 slides inthe cylinder 27 and has three lands 35, 36, and 37 connected by taperedportions 38 and 39. The lands 35 and 37 define working spaces 41 and 42at the two ends of cylinder 27. The working space 42 in addition toreceiving hydraulic liquid at pressure also contains a pair of loadingsprings 43 and 44. The loading spring 43 is retained on the spool member34 so as to permit a small degree of movement thereof before spring loadin either direction is applied from the spring 43. The spool 34 islocatable in either of the limits of free movement by means of a pawl 45engaging a flange 46 on the spool 34. The spring 44 is so arranged, thatafter the spool 34 has moved a predetermined distance in eitherdirection when compressing spring 43, to add a further spring loadresisting further movement of the spool 34. The force required to movethe spool 34 is effected by hydraulic pressures fed to the workingspaces 41 and 42. The hydraulic motor 3 is connected by pipes 47 and 48to the ports 29 and 32 so that depending on the position of spool 34,flow from pipe 24 may pass to either of the two pipes 47 or 48. Thereturn flow from the motor which flows along either pipe 47 or 48 will,depending on the position of spool 34, enter either of the ports 28 or33. These ports are connected together by a pipe 49 for connection to abraking valve 51. A pair of auxiliary ports 52 and 53 open into thecylinder 27 on either side of the port 31 and are alternatively closableby the land 36. The ports 52 and 53 are connected together externally ofthe throttle valve 25 to the pipe 22 which is connected to variousvalves within the circuit. In particular, the pipe 22 connects throughrestrictor 23 to the working space 17. I

The braking valve 51 comprises a cylinder 54 having a pair of ports 55and 56 formed therein and a spool 57 slidable in the cylinder. The spool57 includes a pair of lands 58 and 59 which control the flow between theport 55 and 56 and which also define a pair of working spaces 61 and 62at the ends of the cylinder 54. The working space 62 includes a spring63 acting on the spool. The working space 61 is fed with liquid by arestricted passage 64 within the spool. Working space 62 is fed withliquid by a restricted passage 65 within the spool.

Pressure control within working space 62 is effected both by a pilotvalve 66 and a pilot valve 67. The pilot valve 66 is a spring loadedvalve and responds to hydraulic pressure from the pipe 49 to connectworking space 62 to drain through pipes 68 and 69. The pilot valve 66will connect working space 63 to drain when pressure in pipe 49approaches 2,000 p.s.i. The pilot valve 67 will connect working space 62to drain through pipes 71 and 72 in response to pressures in pipe 22above about 250 p.s.i. The reduction of pressure is working space 62 byoperation of either of the pilot valves 66 or 67 opens a connection invalve 51 between ports 55 and 56 permitting return flow liquid from themotor from port v28 or 33 to pass through to port 56.

A boost pressure valve 73 comprises a cylinder 74 having three ports 75,76 and 77 formed therein and a spool 78 slidable therein. The spool 78includes two lands 79 and 81 which control the connection between port76 to either of the ports or 77. The spool 78 is acted upon by a spring82 to urge it against the force in a small working space 83 formed by anauxiliary piston-and-cylinder and connected to the pipe 22. The port 75receives liquid from pipe 6 at pump delivery pressure whilst the port 77is connected to reservoir 5. Port 76 is connected to port 56 of brakingvalve 51. When the pressure in working space 83 is high the spool 78will move to connect port 76 without restriction to port 77 therebyallowing return liquid pass through valve 51 to pass directly toreservoir. When the pressure in working space 83 is low, e.g., about 250p.s.i. port 77 is closed and there is a restricted connection betweenports 75 and 76 allowing a small make up flow of liquid to pass throughports 76 and 56 and through a pair of non-return valves 84 and 85 intoeither of ports 29 or 32 of the throttle valve 25.

A direction selecting valve 86 is controlled in position by a manuallever 87 having positions corresponding to forward, neutral and reverse.The valve 86 comprises a cylinder 87 having a spool 88 thereincomprising three lands 89, 91 and 92. The lands 89 and 91 control ports93 and 94 which are connected through pipes 95 and 96 to the workingspaces 41 and 42 of the throttle valve 25. A port 97 enters the cylinder87 between the ports 93 and 94 and depending on the position of thespool 88, port 97 is connected to either of the ports 93 or 94. A detentflange 98 on the spool 88 is engageable by a pawl 99 controlled by apiston-and-cylinder unit 101. The piston-and-cylinder unit isspring-loaded so that as the spool disengages from the flange 98 it isconnected through pipes 102 and 103 to respond to the difference inpressures between the working spaces 61 and 62 of the braking valve 51.A small pressure difference only, for example about 25 p.s.i., issufficient to urge the pawl 99 inwardly to engage flange 98.

The lever 87 controls the spool 88 through the medium of a caged spring104.

A master piston-and-cylinder unit 105 is formed by a piston 106 slidablein a cylinder 107 against a spring 108. The piston 106 is urged intocylinder 107 by means of a servo piston 108 slidable within a cylinder109. Cylinder 109 receives liquid at reduced pressure through pipe 111.A rod 112 extends between the pistons 108 and 106 through a reservoirchamber 113. A priming passage 114 extends from the reservoir chamber113 into cylinder 107 at a position where it is just opened by piston106 when in its fully retracted position. A bleed passage 115 extendsthrough piston 108 and rod 112 to open into the reservoir chamber 113,the opening being controlled by a sleeve 116 slidable on rod 112. Theposition of sleeve 116 is adjusted by means of a foot pedal 117, thesleeve 116 being restored to its right hand position as shown by meansof a spring 118 acting on the pedal 117.

A pressure reducing valve 119 receives liquid at pressure from the pumpdelivery connection 6 and by virtue of conventional spring actionsupplies reduced pressure through restrictor 121 to a port 122 openinginto the cylinder 87 of selector valve 86. An internal passage 123within the land 92 of the selector valve connects port 122 to reservoirwhen lever 87 is in the neutral position thus ensuring that when lever87 is in the neutral position no servo liquid at pressure can bedelivered to servo cylinder 109.

The servo motor for adjustment of displacement of the motor 3 is shownat 124. This servo motor is of the differential area piston typecomprising a stepped cylinder 125 within which a stepped piston 126 isslidable providng a pair of working spaces 127 and 128 of which theworking area on the piston of space 127 is one half the working area onthe piston of space 128. A control cylinder 129 within the pistoncarries the servo valve 131 provided with lands so as to connect workingspace 128 either through a restricted passage 132 to the working space127 or to drain through a central passage 133 within the spool valve 131and passage 134 extending from the end of the piston. A spring 135within cylinder 129 urges the spool 131 in a downward direction. Thepiston 126 further defines with its stepped bore a working space 136connected by pipe 137 with the master cylinder 107. The working space127 is fed with high pressure liquid from the motor connections 47 and48 by virtue of two non-return valves 138 and 139 which select thehigher pressure from the motor connections to feed to the working space127.

The motor servo 124 also includes an override valve 141 whose functionis to cause the servo motor 124 to move over-ridingly to a larger motordisplacement when the pressure exceeds 3,000 p.s.i. The valve 141 is fedwith high pressure liquid from the working space 127 of the servothrough a pipe 142, such pressure being fed to act on a spool 143against the compression of a spring 144. If the pressure exceeds apredetermined value, say, 3,000 p.s.i., causing the spool 143 to moveagainst the spring 144, the movement of the spool will connect a pipe145 extending from the working space 128 to a pipe 146 extending toreservoir 5. The control pressure in the space 128 will cause movementof the servo piston to over-ridingly increase motor displacement untilthe pressure in working space 127 reduces below the level, i.e., 3,000p.s.i., to which the valve 141 responds.

Reference is now made to FIG. 3 of the drawings, which illustrate theby-pass circuit. The flow from pipe 26 enters an open-centre controlvalve unit 151 and liquid flowing in the bypass circuit may be selectedby the open-centre valve unit to operate the lift-jack 152 of thefork-lift truck and the tilt-jack 153. By-pass liquid flowing from thepipe 26 will for the most part leave the open-centre valve unit 151through a pipe 154 to enter a flow-sensing unit 155, such flow leavingthe flowsensing unit through a pipe 156 to flow through a cooler 150 toreservoir 5. A signal pressure developed at the flow-sensing unit 155 isfed through pipe 157 to an engine speed control unit 158 which reacts onthe engine speed governor 159 forming part of the engine The open-centrecontrol valve unit 151 comprises, a pair of open-centre control valves161 and 162 having separate manually operable levers 163 and 164respectively. The lift-jack 152 which is controlled by the valve 161 isa single-acting service, that is to say, hydraulic liquid is fed to onlyone working space 163 of the jack through a pipe 164. The weight of thefork-lifting apparatus on its own is normally sufficient under theaction of gravity always to be capable of returning the lift-jack 152when its working space is connected to reservoir. The tilt-jack 153 is adouble-acting jack having a pair of opposed working spaces 165 and 166fed respectively through pipes 167 and 168. An any instant when liquidat pressure is supplied to one of these working spaces to increase itsvolume, then return flow liquid must flow from the other working space.

The control valve unit 151 is of conventional structure, each of thecontrol valves 161 and 162 sliding respectively in cylinders 169 and171. Each control valve 161 and 162 includes a caged spring unitrespectively 172 and 173 whose function is always to tend to restore itsvalve to a neutral position against deflection manually in eitherdirection from the neutral position. Whilst the valve cylinders 169 and171 both include exactly the same port formation, the valves 161 and 162are themselves of slightly differentstructure to accommodate thesingle-acting and double-acting nature of the services. The valve 161includes three lands 174, 175 and 176 which co-operate with five ports177, 178, 179, 181 and 182. The pipe 26 connects to both ports 179 and181. In the neutral position of the valve 161, the port 181 connects tothe port 182 from which the open-centre passage 183 extends to the valve162. The port 178 is connected by pipe 164 to the working space 163 ofliftjack 152. The port 177 is connected through pipe 184 and pipe 156directly to reservoir through the cooler 150. The lands 174, 175 and 176are so located that during movement from the neutral or hold position toselect upward movement of the lift-jack, the land 175 will open aconnection between ports 178 and 179 a little before it closes 181 fromthe port 182 so that the operator by carefully operating the handle maythrottle the flow from port 181 to 182 thus generating pressure in theby-pass circuit, such pressure being connected via ports 179, 178 to theworking space 163 to raise the lift-jack. Movement of the valve 161 tolower the liftjack makes no alteration in the connections in the neutralposition other than to connect port 178 to 177 by movement of land 174,a variable throttling effect being obtained depending on the degree ofopening.

The valve 162 comprises, four lands 185, 186, 187 and 188 whichco-operate with eight ports 189, 191, 192, 193, 194, 195, 196 and 197opening into the cylinder 171. Port 189 is directly connected to thereturn line 184. Port 191 is connected to the working space of jack 153.Port 192 is directly connected to the port 179, port 193 is connected topipe 154 carrying the by-pass flow, port 194 is internally connected topassage 183 and to port 182, port 195 is internally connected to theby-pass pipe 26, port 196 is connected through pipe 168 to working space166 of the tilt-jack and port 197 is connected directly to the returnpipe 156.

Movement of valve 162 in either direction from its neutral position willinitially connect the working spaces 165 and 166 in one sense or theother to the pipe 26 and to the pipe 156, further movement then actingto throttle the flow between passages 193 and 194. The lands and 186control the connection of working space 165 to either of the pipes 26 or156 through either of the ports 192 or 189. The lands 187 and 188control the connection of the working space to either of the pipes 26 or156 through the ports or 197. The lands 186 and 187 irrespectively ofthe movement of the valve from neutral will throttle and close theopen-centre passage between ports 193 and 194. It will be seen thatwhatever direction of selection is applied to the valve 162 to move thetilt-jack, the return flow from the tilt-jack pass through the pipe 184to reservoir without passing through the flow-sensing means 155. Forselection of movement of either the lift-jack or the tilt-jack, thecontrol exerted by the operator is normally such that the by-pass flowpassage from pipe 26 through to pipe 154 will not be completely closed,the flow which is then permitted to pass through the flow-sensing devicebeing such as to enable the operator to control the speed of the engine.

The flow-sensing means comprises a spring-loaded check valve 198parallelly connected with a restrictor 199 whereby liquid flowingthrough pipe 154 may pass to the reservoir through valve 198 andrestrictor 199 jointly into the pipe 156. In the present embodiment thespring-loading of the valve 198 is such that a pressure of 200 p.s.i. isnecessary before the check valve will open. For flow rates from pipe 154through restrictor 199 which do not demand a pressure drop at therestrictor greater than 200 p.s.i. the flow to reservoir will becompletely through the restrictor providing a normal square law relationbetween flow rate and pressure drop. The pipe 157 carries the pressuresignal upstream of the check valve 198 to the engine speed controller158 which comprises a closed container 201 having a plunger 202 slidablethrough a sealed opening in a wall thereof, movement of the plungerbeing opposed by a spring 203. The plunger 202 externally of the casing201 is connected to adjust the setting of the engine speed governor 159.Spring loading of the plunger 202 is so arranged that at a low pressureof about 50 p.s.i. in the casing the spring will move the speed settingto maximum, the engine speed being reduced to half speed as the pressureincreases up to about 150 p.s.i. At 150 p.s.i. the shoulder 204 onplunger 202 will engage the interior of the casing to prevent furthermovement. Further increase in flow rate in pipe 154 will cause rise inpressure drop at the restrictor up to 200 p.s.i. at which point thecheck valve 198 will carry the further flow preventing the pressure dropfrom further increase.

Various functions for the transmission on the fork-lift truck will nowbe described.

Forward Propulsion In order to propel the vehicle forwardly thedirection lever 87 is moved to the appropriate forward position, movingthe spool 88 to the left as seen in the drawing thus connecting themaster cylinder 107 through pipe 137, ports 97 and 93 of valve 86 andpipe 95 to the lefthand working space 41 of the throttle valve 25.Assume the engine is running at its normal speed. In order to propel thevehicle the pedal 117 is then depressed from the position A towardsposition B. The fact that the engine is running and the pump deliveringmeans that liquid under pressure is supplied through pipe 6 to thepressure reducing valve 119 and the selection of forward on lever 87will dis-connect the reservoir passage 123 from port 122 thus ensuringthe delivery of liquid at reduced pressure to the servo motor 109.Depression of the pedal will move sleeve 116 to cover passage 155 sothat pressure is developed against the servo piston 108 to move it tothe-left, moving master piston 106 into cylinder 107 to cutoff port 114and thus to pressurise the liquid in the master cylinder. This liquid istransferred through pipe 137 to working space 41 urging the spool-34 tothe right, firstly displacing the locating pawl 45 and then slightlycompressing the first spring 43. At this position, the land 36 willuncover the auxiliary port 53 and will open a restricted connectionbetween ports 31 and 29, so that high pressure liquid from the pumpthrough pipe 24 may flow through port 29 into motor connection 47 thuscausing the motor to rotate. The dividing valve 7 will receive in itsworking spaces 17 and 18 the pressures respectively from auxiliary port53 and from port 31 such forces urging the spool 8 to the right as seenin the drawing to partially open the flow from port 12 into port 11 andalso to partially close a throttle path from port 12 into port 13. Thisaction will provide a flow from the pump delivery into the ports 11 and13, the proportion between these two flows being dependent on thepressure drop from port 31 to port 29. The adjustment will be such thatthe pressure drop is maintained at a constant value depending on theload of the spring 21. In other words, the flow to the motor will be indirect proportion to the movement of the throttle valve spool 34 whichin turn is in proportion to the pressure generated in the mastercylinder 107. Up to position B for the pedal, a substantial proportionof the liquid delivered by the pump 2 and divided by the valve 7 willflow through the by-pass circuit to and through the valve unit 151 andthrough pipe 154 into the flow-sensing means. The substantial flow willensure that the check valve 198 is open and a maximum pressure drop isdeveloped which when fed to the engine speed controller 158 will urgethe plunger 202 outwardly to the limit thus setting the engine governorat half speed.

Return flow from the motor passes along pipe 48 to port 32 of thethrottle valve and then into port 33. From port 33 the flow will enterpipe 49 and port 55 of the braking valve 51. The pilot valve 66 willalso receive the return pressure from pipe 49 which will be quite low,i.e., considerably less than the pressure at which the pilot 66 willstart to vent working' space 62. The pressure from the auxiliary port 53which is the pressure in the pipe 47 carrying flow to the motor, willreact on the pilot valve 67 causing full opening thereof so that theworking space 62 of braking valve 51 is vented directly to reservoir.Thus the pressure in the return flow of liquid in port 55 iscommunicated by restricted passage 64 to working space 61 and acts onthe spool 57 urging it to the right against its spring 63 and connectingport 55 to port 56 without further restriction. From port 56 the returnliquid enters the port 76 of the boost pressure valve 73. The workingspace 83 of valve 73 will receive liquid at high pressure from pipe 22urging the spool 78 to the right as seen in the drawing providing asubstantially unrestricted connection from port 76 into port 77 and soto reservoir.

Assume now that the driver wishes the vehicle to go faster. He willdepress pedal 117 to the B position which increases pressure in themaster cylinder to cause movement of the throttle valve spool 34 to theright to the extent that the first spring 43 is fully compressed andengagement in just made to start compressing the spring 44. However thiswould demand very substantial increase in pressure in working space 41which will not be available at position B. The driver depresses slightlybeyond position B and the pressure developed in the master cylinder 107and acting in the space 136 of the servo motor 124 will now besufficient to move the pilot valve 131 against the load of its spring135, thus connecting high pressure to the working space 128 causing theservo piston 126 to move upwardly as seen in the drawing to reduce motordisplacement. The flow rate of liquid fed to the motor is thatdetermined by the throttle valve at the position where it is just aboutto start compressing spring 44 and increase in speed is obtained byvirtue of the fact that the motor displacement is reduced whilst theliquid flow through it is maintained constant. As the driver pushes thepedal 117 from position B to position C the liquid displaced from themaster cylinder 107 will cause displacement of the pilot spool 131 sothat the servo piston 126 must follow. At any point, if the driver doesnot move the pedal further, the pilot valve 131 will stop moving andaccordingly the servo piston 126 will stop moving. Within this operationthe engine still rotates at half speed and the by-pass flow through theflowsensing means is sufficient to hold the engine speed at half speed.

Assume now that speed has increased to the extent that the motor is atminimum displacement. This will correspond to position C for the pedal.Again assume that the driver wishes to go faster, he will then depressthe pedal from position C to position D. The servo piston 126 will be onits minimum displacement stop and the pilot-valve 131 will also be heldagainst thestop in its cylinder so that no further increase in volume ofworking space 136 is possible. The further movement of the pedal 117will therefore increase pressure in the master cylinder to the extentthat the pressure increase in the working space 41 will cause the spool34 to move against compression of both springs 43 and 44. This movementwill reduce the restricting effect between ports 31 and 29 and thedividing valve 7 in its attempt to maintain a constant pressure dropbetween these ports will cause a greater proportion of pump delivery topass through the throttle valve to the motor. In turn this means thatthe bypass flow through the by-pass circuit and through the flow-sensingmeans will become smaller and just at the position C for the pedal 117the by-pass flow will reduce to the extent that the pressure at theflow-sensing means 155 will drop below 150 psi. whereby the spring 203will act on plunger 202 to increase the speed setting of the enginegovernor. Thus movement of the pedal from position C to position D willadjust the restricting effect between ports 31 and 29 demanding increasein flow to the motor 3 which in turn will cause a reduction in by-passflow. This reduction in by-pass flow as detected by the flow-sensingmeans will cause an increase in engine speed, the increase in enginespeed acting in effect to maintain flow in the by-pass circuit which atleast will maintain the pressure at the flow-sensing means at 50 psiPressure Over-ride Control In during propulsion at any instant, thepressure fed to the motor attains a value in excess of a pre-setmaximum, e.g., 3,000 p.s.i., the pressure limit valve 141 will respondby connecting the working space 128 of the servo to reservoir. The highpressure in the working space 127 of the servo wil then move the servopiston in a motor displacement increasing direction, although thepressure in the working space 136 holds the pilot valve 131 fullydepressed. This over-riding movement of the piston 126 will cause liquidto be displaced from the working space 136 to increase pressure in boththe master cylinder 107 and in the working space 41 (assuming forwardpropulsion is selected). The increased pressure in the master cylinder107 will not be felt on the pedal 117 because the master piston 106 isservo operated by virtue of sleeve 116. However, the increased pressurein the working space 41 will cause further movement of the spool valvemember 34 to the right against the joint loading of both springs 43 and44 to increase the selected flow from pump 2 to the motor 3. Suchincrease in selected flow may involve reduction in by-pass flow to theextent that the pressure at the flow-sensing means 155 will drop below150 p.s.i. and the engine speed will be increased to help to supply theincreased flow rate. In this way substantially the same speed of thevehicle may be retained up to the maximum engine speed but the operatingpressure of the liquid is maintained constant at say 3,000 p.s.i.

Braking When the driver wishes to reduce speed of the vehicle, he willmerely raise the pedal 117 an amount in accordance with the speedreduction desired. The basic action is independent of the actualposition of the pedal and is as follows. The raising of the pedal willeither reduce the flow rate selected by the throttle valve 25 orincrease motor displacement, such operation immediately serving to makethe motor rotate at a speed greater than that dictated by the flow rateof liquid to the motor. There will be an instantaneous pressure reversalin the motor connections, the connection 47 carrying liquid to the motorsuddenly dropping to a low pressure, and the connection 48 carryingliquid from the motor suddenly increasing to a high pressure. The flowpath for the return liquid from pipe 48 is through ports 32 and 33 ofthe throttle valve, pipe 49, ports 55 and 56 of the braking valve andthrough non-return valve 84 into the port 29 leading to the flow pipe47. Thereby a closed circuit is formed for the liquid flowing throughthe motor and substantial pressure is lost in this circuit in flowbetween the ports 55 and 56. The pilot valve 67 by virtue of lowpressure from pipe 22 is not operative to vent liquid from working space62 of the braking valve, but the pilot valve 66 which now receives highpressure from the pipe 49 will permit a restricted flow of liquid fromthe working space 62 in the sense to ensure that a pressure drop ofsome2,000 psi. will occur when liquid passes from port 55 to the port 56.This closed circuit within which liquid flows to and from the motor,will tend to lose liquid due to leakage and therefore liquid must be fedinto the circuit to prevent cavitation. This function is performed bythe boost valve 73 which on receipt of the comparatively low pressurefrom pipe 22 during braking, will slightly move the spool 78 to theextent to provide a restricted connection between the pump delivery pipe6 through port 75 and into port 76 for feeding to the port 56, which isthe position in the closed motor circuit having the lowest pressureduring braking. This function will ensure that liquid at the pumpdelivery pressure during braking is fed into the closed motor brakingcircuit.

Prevention of Selecting Change in Direction Whilst Vehicle In MotionWhen the vehicle is in motion in a particular direction, the lever 87will be in the forward or the reverse position in which the reservoirpassage 123 is cut off from port 122 and in which the lockingpiston-andcylinder unit 101 is energised from the pressure drop at thebraking valve to urge the pawl 99 to engage one or other side of theflange 98. Whilst the vehicle is in motion, the return flow of liquidfrom the motor through the braking valve, whether during braking orpropulsion will produce a small pressure drop between the two workingspaces 61 and 62, this pressure drop acting on the piston-and-cylinderunit 101 against its light spring loading to hold pawl 99 in engagementwith flange 98. Only when the vehicle has actually stopped movement andthe motor has stopped rotation, does the pressure difference between theworking spaces of the braking valve vanish, and at this instant thespring loading will withdraw the pawl 99 allowing selection of thealternative direction. If before the vehicle has stopped motion, thedriver attempts to select the alternative direction of motion, he couldjust succeed by moving lever 87 to cause the reservoir passage 123 tocoincide with port 122, thereby removing the pressure from the pipe 111.in this case the master piston 106 will immediately retract completely,and the action will be to cause vehicle braking to bring the vehicle toa complete halt. The pawl 99 will then remove and the alternativedirection of propulsion can be selected.

Thus the operations of propulsion and braking have been described forone direction of propulsion only it will be appreciated that similaroperations take place when the alternative direction of propulsion isselected.

Selection of Lift.

When it is desired to operate the forks of the truck to raise a load,the driver will operate the handle 163 of valve 161 towards the liftposition, such movement initially throttling the by-pass flow from pipe26 and through port 181 to 182. The same movement will also connect theport 179 to the port 178 so that liquid may flow from the by-pass pipe26 to the working space 163 of the lift-jack. The greater the throttlingof the by-pass passage by land 175, the greater will be the pressuregenerated and the smaller the flow through the by-pass circuit into theflow-sensing means 155. If the remaining flow through the flow-sensingmeans becomes sufficiently small, the engine speed will be increased bythe speed control unit 158. The operation of valve 161 to select liftmay be either whilst the truck is being propelled or whilst it isstationary. If the lift is selected whilst the truck is stationary,there is no flow to the propulsion motor 3 and the dividing valve willbe in position to close the flow path between ports 11 and 12 leavingthe flow path between ports 12 and 13 fully open. Thus without theselection of lift the pump is substantially unloaded, the deliverypassing completely through the by-pass circuit at low pressure.Selection of lift on the valve 161 will throttle the by-pass circuit andat the same time connect the jack working space 163 to'the pipe 26through ports 178 and 179. The liquid in the pipe 26 is thus pressurisedand the jack 152 will lift. The speed of the engine is controlled by thedriver by virtue of the throttling effect exerted on the by-pass circuitby the valve 161. If the liquid permitted to flow in the by-pass circuitis reduced to a very small amount, the flow detecting means will causethe engine speed to increase. If the selection oflift is made whilst thetruck is moving, there will be pressure in the pump delivery circuit forfeeding to the propulsion motor. The throttling of the by-pass circuitby valve 161 to produce pressurisation in pipe 26 will cause thedividing valve 7 to move slightly'to reduce the throttling effectbetween ports 12 and 13 thus providing pressure to operate thelift-jack.

When it is desired to lower the lift jack, the handle 163 is moved tothe appropriate position causing upward movement of the valve 161 toinitially open a connection between ports 177 and 178 so that theworking space is connected to reservoir through ports 178 and 177, andpipes 184 and 156. This flow does not generate a signal at theflow-sensing means. The land spacing on valve 161 is such that thereturn flow between ports 177 and 178 is opened well before the land 176can start to throttle the by-pass circuit by closing the flow betweenthe ports 181 and 182.

Lowering of the lift-jack may be accomplished when the vehicle isstationary or in motion. Control of the rate of fall being dependentsolely on the adjustment by the driver of lever 163.

Adjustment of Tilt-Jack For adjustment of the tilt-jack the driver willmove the handle 164 to the appropriate tilt forward or tilt backposition. The initial movement of the valve 162 is to connect one orother of the working spaces 165 and 166 to receive liquid from theby-pass circuit. Working space 165 would receive liquid from the port191 and the working space 166 would receive liquid from port 196.Further movement of the valve 162 after connecting the appropriateworking space to the by-pass circuit is to throttle the by-pass circuitso that pressure may be generated for movement of a tilt-jack. Returnflow liquid from the working spaces 165 and 166 will flow either throughport 189 or port 197 into the pipe 184 to return directly to reservoirwithout passing through the flow-sensing means 155. Liquid at pressurerequired from the tilt-jack 153 is very small and normally movement ofthe valve 162 to adjust the tilt-jack will not cause any substantialreduction in flow rate through the by-pass circuit into the flow-sensingmeans 155. The selection of any movement for the tilt-jack 153 will ofcourse demand pressurisation of the flow in the pipe 26 from dividingvalve 7 and as previously described as with respect to the lift-jack 152such pressurisation of pipe 26 will cause a slight adjustment of thedividing valve 7 in order to provide such increased pressure if thevehicle is in motion.

Whilst the described embodiment of the invention has provided a pair ofopen-centre selector valves 161 and 162 capable of using the by-passflow from the dividing valve 7 to operate the services it will beappreciated that within the scope of the present invention there neednot be any provision for operating the services in the by-pass circuit,the flow detecting means being solely in the by-pass circuit and actingto control engine speed. i

I claim:

1. A power comprising assembly comprsing a positive-displacement pump, avariable speed power source adapted to drive the pump, apositive-displacement motor fed with hydraulic liquid delivered by thepump, a variable by-pass to cause some of the hydraulic liquid deliveredby the pump to by-pass the motor in order to vary the motor speed, andmeans for controlling the speed of the power source, said means beingresponsive to the by-pass flow and so arranged that a reduction in theby-pass flow will cause an increase in the speed of the power source andvice versa.

2. A power supply assembly as claimed in claim 1 wherein the pump is offixed positive-displacement.

3. A power supply assembly as claimed in claim 1 wherein the motor is ofvariable positive-displacement.

4. A power supply assembly as claimed in claim 1 wherein the variableby-pass comprises a variable dividing valve and a variable throttlevalve, the dividing valve acting to by-pass part of the pump delivery toa by-pass circuit in the sense to maintain a constant pressure dropacross the throttle valve through which liquid flows to the motor.

5. A power supply assembly as claimed in claim 4 wherein the by-passcircuit includes a flow sensing means for generating a pressure drop asa result of flow of by-pass liquid therethrough, and the means forcontrolling the speed of the power source comprises a spring-loadedvariable volume device connected to respond to pressure drop across theflow sensing means for adjustment of power source speed.

6. A power supply assembly as claimed in claim 5 wherein the flowsensing means comprises a restrictor.

7. A power supply assembly as claimed in claim 5 wherein the flowsensing means comprises a restrictor and a spring-loaded check valve inparallel connection.

8. A power supply assembly as claimed in claim 4 wherein the by-passcircuit includes one or more open centre control valves and a serviceconnected to the or each open centre valve such that the open centrevalve may either permit unrestricted by-pass flow in the bypass circuitor may throttle the by-pass circuit to direct liquid at pressure to theservice.

9. A power supply assembly as claimed in claim 8 wherein the flowsensing means is placed downstream of the open centre valve and meansassociated with the open centre valve to divert return flow from theservice away from the flow sensing means.

10. A hydraulic power transmission comprising a fixedpositive-displacement-pump, a variable-positivedisplacement-motor, afirst slave piston-and-cylinder unit arranged to adjust motordisplacement, a main control, a variable by-pass for selectivelyby-passing a portion of pump delivery liquid, the remainder being fed tothe motor, a second slave piston-and-cylinder unit to control the saidvariable by-pass, a master piston-and-cylinder unit adjustable by saidmain control to displace liquid to the slave piston-and-cylinder units,mechanical means operative on liquid displaced from the masterpiston-andcylinder unit to cause movement of the slave units in adesired sequence, an adjustable speed power source arranged to drive thepump, and flow responsive means to receive liquid bypassed from the pumpdelivery and to control power source speed in the sense that reductionofby-pass flow through said flow responsive means will cause increase inengine speed and vice versa.

11. A hydraulic power transmission as claimed in claim wherein thesequence of slave piston-andcylinder operation provided by themechanical means comprises firstly, adjustment of the variable by-passby the second slave piston-and-cylinder unit to increase flow to themotor, secondly reduction of motor displacement by the iirst slavepiston-and-cylinder unit to increase motor speed and thirdly, furtheradjustment of the variable by-pass by the first piston-and-cylinder unitto increase flow rate to the motor by reducing bypass flow to the flowresponsive means to increase power source speed.

12. A hydraulic power transmission as claimed in claim 11 including apressure responsive means responding to receive hydraulic pressurewithin the motor to overridingly increase motor displacement and toreact on the first slave piston-and-cylinder unit to increase pressuretherein and thereby to reduce the bypass flow to cause adjustment of theflow adjusting means to increase power source speed.

13. A hydraulic power transmission as claimed in claim 12 wherein themechanical means comprises spring loading, one stage of spring-loadingacting on the first slave piston-and-cylinder unit to provide saidsecond adjustment and two stages of spring loading acting on the secondslave piston-and-cylinder unit-t0 provide said first and thirdadjustments.

1. A power comprising assembly compRsing a positive-displacement pump, avariable speed power source adapted to drive the pump, apositive-displacement motor fed with hydraulic liquid delivered by thepump, a variable by-pass to cause some of the hydraulic liquid deliveredby the pump to by-pass the motor in order to vary the motor speed, andmeans for controlling the speed of the power source, said means beingresponsive to the by-pass flow and so arranged that a reduction in theby-pass flow will cause an increase in the speed of the power source andvice versa.
 2. A power supply assembly as claimed in claim 1 wherein thepump is of fixed positive-displacement.
 3. A power supply assembly asclaimed in claim 1 wherein the motor is of variablepositive-displacement.
 4. A power supply assembly as claimed in claim 1wherein the variable by-pass comprises a variable dividing valve and avariable throttle valve, the dividing valve acting to by-pass part ofthe pump delivery to a by-pass circuit in the sense to maintain aconstant pressure drop across the throttle valve through which liquidflows to the motor.
 5. A power supply assembly as claimed in claim 4wherein the by-pass circuit includes a flow sensing means for generatinga pressure drop as a result of flow of by-pass liquid therethrough, andthe means for controlling the speed of the power source comprises aspring-loaded variable volume device connected to respond to pressuredrop across the flow sensing means for adjustment of power source speed.6. A power supply assembly as claimed in claim 5 wherein the flowsensing means comprises a restrictor.
 7. A power supply assembly asclaimed in claim 5 wherein the flow sensing means comprises a restrictorand a spring-loaded check valve in parallel connection.
 8. A powersupply assembly as claimed in claim 4 wherein the by-pass circuitincludes one or more open centre control valves and a service connectedto the or each open centre valve such that the open centre valve mayeither permit unrestricted by-pass flow in the by-pass circuit or maythrottle the by-pass circuit to direct liquid at pressure to theservice.
 9. A power supply assembly as claimed in claim 8 wherein theflow sensing means is placed downstream of the open centre valve andmeans associated with the open centre valve to divert return flow fromthe service away from the flow sensing means.
 10. A hydraulic powertransmission comprising a fixed positive-displacement-pump, avariable-positive-displacement-motor, a first slave piston-and-cylinderunit arranged to adjust motor displacement, a main control, a variableby-pass for selectively by-passing a portion of pump delivery liquid,the remainder being fed to the motor, a second slave piston-and-cylinderunit to control the said variable by-pass, a master piston-and-cylinderunit adjustable by said main control to displace liquid to the slavepiston-and-cylinder units, mechanical means operative on liquiddisplaced from the master piston-and-cylinder unit to cause movement ofthe slave units in a desired sequence, an adjustable speed power sourcearranged to drive the pump, and flow responsive means to receive liquidby-passed from the pump delivery and to control power source speed inthe sense that reduction of by-pass flow through said flow responsivemeans will cause increase in engine speed and vice versa.
 11. Ahydraulic power transmission as claimed in claim 10 wherein the sequenceof slave piston-and-cylinder operation provided by the mechanical meanscomprises firstly, adjustment of the variable by-pass by the secondslave piston-and-cylinder unit to increase flow to the motor, secondlyreduction of motor displacement by the first slave piston-and-cylinderunit to increase motor speed and thirdly, further adjustment of thevariable by-pass by the first piston-and-cylinder unit to increase flowrate to the motor by reducing by-pass flow to the flow responsive meansto increase power source speed.
 12. A hydraulic power transmission asclaimed in claim 11 includIng a pressure responsive means responding toreceive hydraulic pressure within the motor to overridingly increasemotor displacement and to react on the first slave piston-and-cylinderunit to increase pressure therein and thereby to reduce the by-pass flowto cause adjustment of the flow adjusting means to increase power sourcespeed.
 13. A hydraulic power transmission as claimed in claim 12 whereinthe mechanical means comprises spring loading, one stage ofspring-loading acting on the first slave piston-and-cylinder unit toprovide said second adjustment and two stages of spring loading actingon the second slave piston-and-cylinder unit to provide said first andthird adjustments.